Implant with reinforced resorbable stem

ABSTRACT

A prosthetic implant for a bone joint has an anchor formed of a resorbable sleeve reinforced with a nonresorbable core. Also disclosed are methods of installation.

This application is divisional application of Ser. No. 07/897,197, filedJun. 11, 1992 for IMPLANT WITH REINFORCED RESORBABLE STEM now U.S. Pat.No. 5,571,193, which in turn is a continuation-in-part of Ser. No.07/850,891 filed Mar. 12, 1992, now U.S. Pat. No. 5,469,492.

BACKGROUND OF THE INVENTION

The prosthetic replacement of joints has evolved over the years fromearly, relatively crude models to current prostheses which closelyreplicate functions and motions of a natural joint. Some of the jointsamenable to prosthetic replacement include the shoulder, hip, knee,ankle and wrist. Prosthetic joints have provided patients with increasedcomfort, range of motion and nearly normal lifestyles.

Some of the problems encountered with prosthetic joints includeexcessive wear between components of the prosthetic implants which movewith respect to each other. Additionally, movement of the implant withrespect to the patient's bone compromises fixation. A third problem isan abnormal stress transference from the implant to the bone.

To address some of the problems of fixing the implant to the bone,cemented implants have been developed. The cement initially acts as agrout to "form fit" the implant to the bone. After the cement cures to ahard material, it becomes mechanically fixed to the bone byinterdigitating into the bone trabeculae. Cementing a plastic or metalimplant securely to natural bone has greatly improved the status ofjoint replacement over the prior state of merely implanting thecomponent into the bone and hoping that it actually stayed securely inplace. Cement fixation generally provides excellent short term results.However, younger, heavier, or more active individuals may find that thebond between cement and bone eventually breaks down. Consequently, aloosening or separation between the bone and the implant occurs.

To address the problem of cement breakage or implant separation, analternative is to coat a surface of the implant with a porous materialwhich allows the patient's bone to grow into the pores, therebymechanically fixing the implant to bone or alternatively coating theimplant with a calcium-phosphate type of ceramic which may chemicallybond the implant to bone. As a result in either case, the implant isbiologically attached to the bone. This "cementless" procedurerepresents the current state of the art in implantation of jointprostheses. The patient's own tissue eventually holds the implantsecurely in place, either mechanically or chemically, and the implantsubsequently becomes a permanent part of the bone.

Another problem, seen with both cemented and cementless implants, isoccasional extensive osteolysis. This osteolysis, or bone dissolution,occurs throughout all areas of the bone into which the stem wasimplanted. The etiology is possibly a histiocytic foreign body reaction.In the case of cemented devices, the reaction may be from fracturedparticulate cement and/or secondary to polyethylene particulate weardebris. In the case of cementless stemmed implants, the reaction is feltto be secondary to particulate polyethylene or particulate metal alloyfrom fretting wear of the implant. Regarding reaction to metal alloy,fretting secondary to micro-motion at the implant-bone interfacetheoretically initiates the foreign body immune response.

A further problem encountered with joint implants is an abnormal stresstransference from the implant to the bone. The ideal stress transferenceof load to the bone is the normal, anatomical transference. Toapproximate it, the implant material should have mechanical propertiessimilar to those of bone and should replace only the destroyed jointsurface. Thus no implant material, or only a minimal amount of implantmaterial, would be placed in the intramedullary canal of the bone. Thisis difficult to do with implants having porous surfaces. The reason isthat such implants require immediate rigid fixation for a sufficienttime period, at least six to twelve weeks, to assure bony attachment orchemical bonding. If the device is not held rigidly, micro-motion occursat the implant-bone interface. The result is a less stable fibroustissue interface rather than the necessary, more stable, securely-fixedbony attachment.

Currently the most common method of holding the implant rigidly in thebone is by providing the implant with a stem. The stem "press-fits" intothe intramedullary cavity of the bone, e.g., the femur. Such a press-fitof the stem into the shaft of the bone holds the device rigidly andallows for an adequate bone attachment or chemical bonding for securefixation. For the surgeon it also provides the desired proper anatomicalplacement of the implant in the bone in a reproducible manner. If nointramedullary cavity is available, as in the pelvis for example, theimplant is anchored to the bone with a threaded anchor bolt.

The shortcoming of the aforesaid approach is that loading of the bone isno longer physiologic. Instead of the normal loading primarily at theend of the bone near the joint surface, the bone is loaded more distallywhere the stem of the implant is affixed. The result is an abnormaltransference of stress which bypasses or "unloads" the end or jointsurface portion of the bone. Consequently that portion of bone undergoesresorption. This leads to weakening over a period of years, thuscreating a potential for fracture or resorption of the bone thatpreviously held the implant securely. The result is again a loosening ofthe implant within the bone with all the adverse consequences previouslymentioned.

For implants which are held in place with a screw, such as an acetabularcup of a hip prosthesis, the non-physiologic transference of stresses isless pronounced. This is because the location and orientation of theanchoring bolt can be selected to minimize non-natural load transferencestresses. Nevertheless, the potential problems remain; namely, frettingwith resultant corrosion and lysis or fatigue failure from micro-motionand eventual fracture of the anchoring bolt.

Because a stem placed down the medullary cavity of the bone produces anabnormal stress distribution, the possibility of using an implantwithout a stem presents itself. Such an implant would essentially onlyresurface the destroyed articular surface. This is more readily done incertain joints, such as the knee, elbow, or ankle, than in others, suchas the hip, shoulder or wrist.

However, even if a stemless implant is feasible, its immediate rigidfixation is not as secure as if the implant were anchored with a stem ora bolt. Because the stem or bolt functions to align an implant in itscorrect position until bone attachment is complete, an alternativemechanism is necessary to accomplish these functions if a stemlessimplant is used. One such mechanism could be transcortical fixation ofthe implant with multiple screws. This, however, makes it more difficultfor the surgeon to correctly and reproducibly position and align theimplant. An alternative approach is disclosed and claimed in U.S. Pat.No. 4,990,161 the entire disclosure of which is made part hereof andincorporated herein by reference.

SUMMARY OF THE INVENTION

To approximate an ideal physiologic stress transference across a bonejoint, the implant stem or anchor should fit tightly into place withrespect to the natural bone. The implant may be porous-coated as well ascoated with a chemical material such as calcium-phosphate ceramics topermit bony attachment and/or chemical bonding therein. Finally, theimplant should not interfere substantially with stress transference inthe long term. One approach to provide these features is found U.S. Pat.No. 4,990,161, the disclosure of which is incorporated herein byreference.

The '161 patent provides an implant with an anchor comprised of abiodegradable material. The biodegradable anchor includes an elongatedmember which has an exterior surface shaped to tightly engage a bonecavity. Because of the tight engagement, the anchor is substantiallyimmovable within the cavity upon implantation. The anchor is made of amaterial which resorbs at preselected time periods after implantation,such as a number of weeks or months up to approximately one year ormore.

Unfortunately, totally resorbable polymers do not necessarily possessthe structural strength and integrity for a load-bearing stem. Toovercome this problem and provide the required strength and rigidity, animplant constructed in accordance with the present invention has acomposite anchor, including a stem having a nonresorbable coresurrounded by a layer of resorbable material. The core increases themechanical strength required for a load bearing anchor, such as in thehip or knee. For long bone or joint implants, the core extends from thepermanent articulating bearing surface component into the medullarycanal.

Initially, the resorbable layer gives secure anchorage of the stem withan interference fit equivalent to that obtainable with the current stateof the art in prosthetic joint replacement. Selected surface portions ofthe permanent implant component are porous to permit and direct bonyattachment. A coating of calcium phosphate may be provided to enhancebony ingrowth and possibly chemically bond the implant to bone.

After an optimal period of time elapses to permit sufficient bonyattachment around the permanent, porous implant component, the polymersurrounding the metal core of the anchor slowly degrades and ultimatelydisappears. As a result, only the small diameter nonresorbable coreremains in the medullary cavity of the bone. Although a small diametercore remains permanently within the medullary canal, no load bearingoccurs through that core because it is spaced apart from cortical boneand, therefore, does not come in contact with the surrounding corticalbone structure. The core becomes essentially nonfunctional and the bonedoes not "realize" that any stem is actually present. In biomechanicalterms only the permanent implant component forming the articulatingsurface of the joint remains as a permanent functional component and itis the only artificial component across which stresses are beingtransferred.

Thus, the present invention provides an implant with an anchor includinga nonresorbable core surrounded by a resorbable sleeve. Thenonresorbable core strengthens and reinforces the sleeve. The core issized so that it can not accept significant load bearing. Consequently,once the sleeve is resorbed, the nonresorbable core is nonfunctional anddoes not impede normal stress transfer.

The invention negates the problem of distal loading of the bone withresultant proximal bone resorption seen with stemmed devices. Problemssuch as those seen in hip replacement arthroplasty are eliminated orminimized. Because the remaining small diameter core is essentially freefloating within the medullary cavity, fretting with potential forosteolysis is no longer a significant potential.

Selected portions of the implant surface intended to contact the boneare porous. Such a surface promotes bony attachment to further anchorthe implant and to approximate the natural mode and stress transferencesituation after bony ingrowth has occurred. After the stem is no longerrequired, the resorbable sleeve disappears by a natural biologicerosion. The resorption of the sleeve material can be predetermined totake place over any desired length of time, i.e, six weeks, eight weeks,twelve weeks, six months, one year or more. These factors will depend onhow much time the particular bone normally takes to adequately attach tothe implant as well as clinical considerations such as the age andoverall health and mobility of the individual.

The nonresorbable core is typically formed from a metal alloy such as atitanium alloy, most preferably titanium-6-aluminum-4-vanadium.Alternatively, other alloys of cobalt-chromium or stainless steel may beemployed. Other materials which can be used for the core includenonresorbable ceramics such as aluminum oxide and zirconia. Thepermanent implant can also be formed from materials such asnonresorbable plastics, e.g., polyethylene or even composite materialssuch as carbon fiber-reinforced polymers such as polysulfone.

Generally, articulating surfaces of the permanent, functional implantare formed of permanent nonresorbable materials such as a metal alloy,ceramic, carbon and polymers. Preferred alloys are titanium, such astitanium-6-aluminum-4-vanadium. Other alloys include cobalt-chromium andstainless steel. Ceramics suitable for permanent articulating surfacesinclude aluminum oxide and zirconia.

The length and diameter of the elongated, composite anchor or stem willvary according to the physical characteristics of the patient.Preferably, diameter variations are achieved by varying the thickness ofthe resorbable sleeve.

Modular construction is most preferred where practical to facilitateinterchange of parts and thereby minimize the inventory of parts thatmust be kept. For example, a head or ball component of a hip replacementis preferably of modular construction to cooperate with a neck portion.Also, stem or anchor portions having an inner permanent core and anouter resorbable sleeve are preferably formed by modular construction.Taper locks are preferably used for interconnecting the parts which makeup a complete implant.

The resorbable sleeve and resorbable anchoring bolts or screws arefabricated from a variety of biodegradable materials. Many examples ofsuch materials are known to the artisan, including certain ceramics suchas calcium hydroxyapatite or tricalcium phosphate, high molecular weightpoly-L-lactic acid (PLLA) polymers, such as that used to fabricatecurrently used bioresorbable screws, adapted to prosthetic implants,other polymers, such as polymers of glycolic acid or lactic acid,polyamides of α-amino acids, polydioxanone, polylactic acid-polyglycolicacid copolymers, polyorthoester, polycarbonate and polyetheretherketone(PEEK). Also, unmodified or modified natural polymers such as gelatin orstarch can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, front elevation view through a human hip joint,illustrating the pelvis and the upper portion of the femur, and shows aprosthetic hip constructed in accordance with the present inventionimmediately after installation;

FIG. 2 is a schematic front elevation view through a human hip jointillustrating an alternate embodiment of a prosthetic hip constructed inaccordance with the present invention;

FIG. 3 is a schematic front elevation view through a human hip jointillustrating a further embodiment of a resurface femoral head componentmaking the 70° inclination concave rather than planar;

FIG. 4 is a schematic front elevation view of a prosthetic kneeconstructed in accordance with the present invention;

FIG. 5 is a cross-sectional view taken through line 4--4 of FIG. 1;

FIG. 6 is a cross-sectional view taken through line 5--5 of FIG. 2; and

FIG. 7 is a cross-sectional view taken through line 6--6 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a total prosthetic hip constructed in accordancewith the present invention is made up of a femoral implant 1A and anacetabular implant 1B. The femoral implant includes a trochanteric bodypiece 2 which has a porous surface 22 over the portion of its exteriorwhich is intended to be directly affixed to the natural bone. To achievethe desired bony ingrowth, pores of the porous surface are preferablysized in a range between about 250-450μ. The exact dimensioning of thepores is known to those skilled in the art and is not particularlycritical. Also included is an anchor 4 which is comprised of a permanentcore 8 and a resorbable sleeve 6. Core 8 is preferably made of a metalalloy such as titanium-6-aluminum-4-vanadium.

Outer resorbable sleeve 6 is formed of a biodegradable material, forexample, high molecular weight poly-L-lactic acid (PLLA) polymers. Othersuch materials include calcium hydroxyapatite, tricalcium phosphate andthe compounds previously mentioned.

In a preferred embodiment, anchor 4 is affixed to trochanteric bodypiece 2 by means of a taper lock 10. Preferably anchor 4 includes afemale portion and trochanteric body piece 2 includes a male portion oftaper lock 10. The male and female portions have cooperating surfaceswhich are tapered at an angle selected so that they become effectivelypermanently secured to each other when the two portions are firmlypushed together. The female portion is preferably formed in anchor 4 soas to prevent formation of stress concentrations in the trochantericbody piece 2. The interconnection between trochanteric body 2 and anchor4 is typically in the region of anchor 4 which straightens out in alateral plane, although it may be located at other sites along anchor 4.

This modular construction of the femoral implant 1A makes it possible tostock an inventory of differently shaped and/or sized components thatcan be assembled to fit any given patient. Thus, resorbable outer sleeve6 has multiple outer diameter sizes to fit different medullary shaftdiameters while inner core 8 has a constant diameter so that the outerdiameter of anchor 4 varies with the thickness of resorbable sleeve 6.In addition, the parts inventory will include anchors of varyinglengths. Thus, the surgeon can select the permanent implant and theanchor which have the desired dimensions and shapes from the availableinventory, assemble them with the above-discussed taper locks, and theninstall the implant in the patient.

In another embodiment the cross section of inner core 8 of anchor 4 isnot rounded, but, for example, a quadrilateral, e.g., square orrectangular. See FIG. 4. By extending the square cross section to taperlock 10 it is possible to predetermine the relative angular orientationof the modular components of the implant. Anchor 4 is inserted by pressfitting into place.

Trochanteric body piece 2 is in continuity with a modular head or ball12, preferably formed of metal alloy such as a titanium alloy.Titanium-6-aluminum-4-vanadium is a preferred alloy. Other suitablealloys include cobalt-chromium and stainless steel. Trochanteric bodypiece 2 is connected to head 12 by a neck 13 which extends medially andupwardly with respect to trochanteric body piece 2. Head 12 cooperateswith an acetabular cup 14 of the acetabular implant 1A.

In one embodiment, neck 13 forms a taper lock 34 with head 12 to securethe two to each other. Head 12 preferably has a female aperture whichcooperates with a male extension of neck 13 to form the taper lock 34.Alternatively, other locking means, such as a threaded screw arrangement(not separately illustrated), can be used.

The acetabular implant includes an inner acetabular component 16 and anouter acetabular component 18. Inner component 16 is preferably formedof polyethylene although other noncorroding, nonresorbable high strengthmaterials can be used. Outer acetabular component 18 is formed oftitanium alloy although other high-strength materials, such as cobaltchromium alloy, stainless steel alloy and ceramics, can be used. Thefemoral head is preferably formed of zirconia.

Inner acetabular component 16 is cup-shaped and has a concave surfacefacing and engaging ball 12 of the femoral implant and permitting freemovement of the ball in the socket. Outer acetabular component 18includes multiple fixation holes, or alternatively a single bore,positioned so that upon implantation the bore is aligned with apectineal line of the patient's pelvis as is described in greater detailin U.S. Pat. No. 4,990,161.

Preferably, resorbable screws 20 are used to secure acetabular cup 14 topelvic bone 35. Alternatively, the holes and bore can include a recessdefining an inwardly facing shoulder that, upon implantation, engages aset screw having a threaded end extending into and engaging screw 20 (asis more fully described in U.S. Pat. No. 4,990,161). The set screw isnonresorbable and acts as an interconnection between resorbable screw 20and nonresorbable outer component 18.

The exterior surface of outer component 18 in contact with the bone ofthe patient's pelvis and where stress transference is to take place hasa porous surface 32 for anchorage of component to bone. Porous surface32 promotes bone ingrowth into the area of load transfer between thepelvis and acetabular cup 14. To achieve the desired ingrowth, the poresare preferably sized in a range between about 250-450μ.

Inner and outer acetabular components 16/18 are held together by meansknown to the artisan. Typically, a lip is provided on one of components16/18 and a key arrangement locks components 16/18 together.

Resorbable screw 20 has a first end including an external thread formedto cut into and firmly engage the bone structure. Resorbable screws 20are typically 6.5 mm in diameter. In a presently less preferredalternative, an opposite free end of screw 20 includes an internalthread which is engaged by a nonresorbable set screw.

Screw 20 is preferably fabricated from a resorbable material such asmodified or unmodified natural polymers, biodegradable ceramics, andother materials known to the artisan. The material is engineered toresorb at a predetermined time. For most applications, this time willcoincide with a time period sufficient to allow mature bony attachmentabout acetabular component 14 for adequate mechanical fixation of theimplant to the pelvis.

During implantation, resorbable screw 20 is aligned with the bone of thenatural acetabular cup. Screw 20 is tightened to firmly secureacetabular cup 14 to the bone. After implantation, inner component 16may be positioned so that an overhanging portion 28 protrudes beyond thenatural cup. This provides better seating of ball or head 12 over awider range of motion and helps prevent accidental dislocations of ballor head 12 and the socket.

To implant the femoral implant 1A into a femur 24, the head and neck offemur 24 are resected and an external femoral loading face is formed.The loading face is planar and generally perpendicular to a pectinealline and at an angle of approximately 70° to a longitudinal axis offemur 24. A cavity is shaped in the intertrochanteric aspect of thefemur 5 by removing soft non-load bearing bone tissue which is mostlymarrow. The entire intertrochanteric cavity may be broached slightlyunder size relative to the implant to achieve a secure press fit.

Trochanteric body 2 has a nonresorbable male taper lock dependingtherefrom. Prior to implantation, anchor 4 is connected to trochantericbody 2 by pressing a portion of taper lock 10 of body 2 into areciprocating portion of taper lock 10 formed in anchor 4.Alternatively, this lock can be a screw connection.

Trochanteric body 2, including anchor 4, is pressed into theappropriately dimensioned intertrochanteric upwardly opening cavity infemur 24 until the internal and external loading surfaces firmly engagethe bone. During the implantation process, elongated anchor 4 slidesinto the medullary canal of the femur and acts as a guide to preventmisalignment of trochanteric body 2 during the insertion. This isparticularly important when substantial force must be applied toovercome the press fit between the body and the cavity.

Resorbable sleeve 6 firmly engages the walls of the medullary canal witha press fit to secure and fix anchor 4 and trochanteric body 2 untilbony ingrowth has attached body 2 to the patient's own femur. Innermetallic core 8 acts as a reinforcement to strengthen anchor 4 andprevent it from cracking, breaking or otherwise deforming. After sleeve6 is totally resorbed, inner core 8 is essentially free floating within,that is, it does not contact the cortical walls of, medullary cavity 26and it is sized such that it does not accept loads.

Once the implant is firmly fixed by bony attachment in its properposition, the function of anchor 4 is over and its functional portion,sleeve 6, is eliminated by resorption. The final result is afunctionally stemless femoral component that allows a near perfectphysiologic transference of stresses to the proximal femur as opposed toa stemmed implant which is fixed distally. Distal fixation, asencountered with permanent stems which remain in contact with corticalbone, creates a potential for stress shielding and bone resorptionproximally with an increased chance of mechanical failure.

After implantation, after proximal bony attachment and after resorptionof outer sleeve 6, there is no significant distal loading on the bone.This is the normal physiologic and anatomic situation. The absence ofsuch distal loading about the femoral cortex adjacent to anchor 4coupled with the transfer of loading and stresses proximally assuresthat the femoral implant is top loaded in a manner analogous to thenormal femur in a healthy hip joint.

FIG. 2 illustrates an alternative embodiment of the femoral implant ofthe present invention. It comprises a resurface femoral head implant 36having an anchor 38. Anchor 38 includes a resorbable sleeve 44surrounding a nonresorbable inner core 46. Resurface component 36 isformed of a nonresorbable material, preferably a metal alloy. Implant 36defines a generally semi-spherical head 37 which is shaped to fit intoand cooperate with an acetabular cup 56 upon implantation. The surgeonselects implant 36 from an inventory of several, perhaps 4 or 5, sizesvarying from small to large.

Resurface component 36 is placed in contact with a patient's transectedfemoral head 64. This transection across the femoral head should beinclined approximately 70° to the longitudinal axis of the femur so thatthe applied loading forces act substantially perpendicular to theimplant surface. Consequently, substantially only compressive forces aregenerated between the implant and the femur with theoretical enhancementand earlier bony attachment over the exterior load transferring implantsurface. This 70° inclination being perpendicular to the implant surfacewould tend to enhance stabilization of the implant following resorptionof the resorbable sleeve. Resurface implant 36 protrudes from thegenerally planar surface of a partially resected femoral head 64. Aporous surface 66 is defined by a portion of the implant which is inintimate contact with the transected femur head 64. Porous surface 66promotes bony attachment of the implant to the femur.

FIG. 3 is an illustration of a further embodiment of a resurface femoralhead component making the 70° inclination concave rather than planar,thus affording greater stability of the implant. Thus, surface 66a iscurved instead of being substantially planar.

Core 46 is preferably formed as one piece with head 37. Alternatively,core 36 can be secured to resurface component 36 by any number of meansknown in the art, e.g., with the above-described taper lock or with athreaded connection. Anchor 38 extends in a lateral and downwarddirection with respect to head 37. That is, a lower or distal surface ofimplant 36 which contacts partially resected femoral head 64 is placedat approximately a 70° cut angle with a longitudinal axis of femur 24.

Anchor 38 is seated tightly into intramedullary canal 26 with apress-fit. Resorbable sleeve 44 may be tapered and dimensioned tointerface the breached bone canal. A cross-sectional view of eitheranchor 38 shows a quadrilateral inner core cross section, mostpreferably of a square or rectangular shape. See FIG. 5.

Anchor 38 is preferably also of modular construction as described aboveso that the size appropriate for a given patient can be selected from aninventory of differently sized and/or shaped anchors. The modules areconnected just prior to implantation with a taper lock, for example. Foroptimal efficiency, the inventory consists of a select number of cores46 of equal diameter but differing lengths and a number of resorbablesleeves 44, all having the same inner diameter so that they fit snuglyonto any of the cores but of different outer diameter so that they canbe press-fit into medullary canals of varying diameters. The optimallength of the combined core and sleeve would be such that it wouldcontact the lateral cortical wall of the femur distal to the greatertrochanter and opposite the lesser trochanter thus further stabilizingthe implant.

Finally, the sleeves will have a sufficient length so that they can befitted onto the largest core. For shorter cores, the sleeve is cut tothe appropriate length just prior to assembly and installation.

Acetabular cup 56 is constructed as described above and includes anouter component 58 and an inner component 60. The outer component isusually formed of a metal alloy, such as a titanium alloy. Innercomponent 60 is preferably formed of polyethylene. Other materials aresuitable as discussed above. The outer component 58 is provided with aporous surface 52 as well as possibly being coated with calciumphosphate to direct and encourage bony attachment as previouslydiscussed.

Outer and inner components 58/60 are held together by means known in theart as discussed above. Resorbable screws 54 hold outer component 58 topelvic bone 35 until bony attachment occurs. Resorbable screws 54 aretypically 6.5 mm in diameter.

Turning now to FIG. 4, a total knee prosthesis constructed in accordancewith the present invention is shown. The prosthesis includes a femoralcomponent 70 and a tibial component 90. Femoral component 70 includes astem 74 having a resorbable outer sleeve 76 and an inner nonresorbablecore 78. Inner core 78 is typically formed of a metal alloy such as atitanium alloy, preferably titanium-6-aluminum-4-vanadium. The stem canbe of a modular construction as previously described. A cross-sectionalview of stem 74 shows a quadrilateral-shaped core dimension aspreviously discussed. See FIG. 6.

Femoral component 70 also includes an articular bearing surface which ispreferably formed of a metal alloy. A selected portion of articularbearing component 72 to be in contact with the natural femur has aporous surface 84 for bony attachment. Articular bearing component 72terminates proximally in a portion of a lock, preferably a taper lock80. Alternatively a screw-type lock may be utilized.

The portion of taper lock 80 of femoral component 72 is preferably afemale portion. Taper lock 80 is designed to cooperate with a distaltermination of stem 74 which includes an opposite cooperating portion oflock 80, preferably a male portion of taper lock 80.

Tibial component 90 includes a stem 92 which is comprised of an innernonresorbable core 96 and a resorbable outer sleeve 94. Inner core 96 isformed of a metal alloy. A preferred alloy istitanium-6-aluminum-4-vanadium although other alloys, such ascobalt-chromium or stainless steel, can be used. Resorbable outer sleeve94 is again preferably of modular construction as is inner metal core96. Inner metal core 96 preferably has a cross-section which isquadrilateral in shape, preferably square or rectangular, to reducepossible slippage or torsion.

Inner metal core 96 terminates proximally in a portion of a taper lock98, preferably a male portion which cooperates with a female terminationof a tibial tray component 88. Tibial tray 88 is nonresorbable and ispreferably formed of a metal alloy such astitanium-6-aluminum-4-vanadium. At a lower or distal surface of tibialtray 88, there is a porous surface 82 for the desired bony attachment.

Proximal to tibial tray 88 is a bearing component 86 which can be madeof polyethylene, ceramic or even a metal alloy. Bearing component 86 is:in contact at its lower or distal surface with an upper or proximalsurface of tibial tray 88.

Bearing component 86 and tibial tray 88 are held together by means wellknown to the artisan. Usually, one of tibial tray 88 or bearingcomponent 86 is provided with a lip. A key arrangement locks the twocomponents together. Resorbable screws 100 anchor tibial tray 88 to thepatient's tibia until bony attachment into the lower surface of tibialtray 88 assures bony fixation.

Bearing component 86 is configured at its upper or proximal surface withtwo cup-shaped or concave portions designed to cooperate with femoralcomponent 72. Femoral component 72 is provided at its lower or distalsurface with projections which are convex and cooperating to adapt withthe configuration of bearing component 86. This arrangement is inimitation of the natural human knee joint. That is, the femoralprojections of femoral component 72 represent and mimic the femoralcondyles and the upper or proximal surface of bearing component 86 isshaped to mimic the articulating surface of the proximal tibia.

In installation, femoral implant 70 is inserted into the patient'sfemoral medullary cavity after resection of the distal femur. Resectedportions include the articulating surfaces. Femoral implant 70 isinserted after opening of a cavity in the femoral shaft. Femoral stemcomponent 70 acts as an alignment and anchoring mechanism for theartificial joint.

In similar fashion, an appropriately dimensioned area in the tibialmedullary shaft is opened. After appropriate resection of the proximaltibial area including the articulating surface, tibial component 90 isplaced into the opening in the patient's tibial medullary shaft. Tibialstem component 90 acts as an alignment and anchoring mechanism for theartificial knee joint.

Femoral articular bearing component 72 has a porous surface 84 which isin intimate contact with the transected natural femur. Similarly, theporous surface 82 of tibial tray component 88 is in intimate contactwith the transected natural tibia. Porous surfaces 84/82 permit anddirect bony ingrowth into the prosthetic devices to permanently anchorthem. After bony attachment has occurred, the resorbable materials,including sleeve 76 of femoral implant 70 and sleeve 94 of tibialimplant 90, resorb over a predetermined time period.

After resorption of sleeves 76/94, remaining inner cores 78/96 arenonfunctional. That is, they are dimensioned such that they do notcontact the bony cortices, thereby accepting no load bearing. Thus, thecores are essentially free floating within the medullary canal. As aconsequence, no significant abnormal stress transference occurs, incontrast to conventional, non-resorbing stemmed devices.

Although hip and knee replacement have been described withparticularity, the present invention is not so limited. Resorbableanchoring devices including a resorbable sleeve reinforced by an innernonresorbable strengthening core are applicable to all joints includingwrist, finger, elbow, ankle, foot, toe and shoulder. The artisan mayreadily appreciate other uses and variations of the present invention.The invention is not to be limited by the aforementioned examples butrather by the scope of the claims which follow.

What is claimed is:
 1. A method for installation of a prosthetic implantcomprising the steps of:a. providing a functional implant componentadapted to remain in permanent contact with a bone; b. forming a portionof the component in contact with the bone so that bony ingrowth canattach directly thereto; c. opening a cavity in the bone; d. placing thefunctional component against the bone; e. providing an anchor having anonresorbable core surrounded by a resorbable sleeve which has a lengthand a periphery formed for firmly engaging the cavity so that a crosssection of the core is sufficiently smaller than a cross section of thecavity so that contact between the bone and the core followingresorption of the sleeve is prevented, the core extending over a majorportion of the length of the sleeve; f. inserting the anchor into thecavity so that the anchor is substantially immovably disposed within thecavity; g. fixedly securing the component to the anchor so that thecomponent is in contact with bone surrounding the cavity; h. growingbone proximate the cavity into the portion of the component to therebypermanently fix the component to the bone; and i. substantiallycompletely resorbing the anchor after the completion of the promotingstep so that natural tissue can grow back into the cavity; whereby thecore remains permanently in the cavity and is spaced from the surfacesof the cavity so that there is substantially no load transferencebetween the bone and the core.
 2. The method of claim 1 wherein the stepof opening the cavity comprises opening a cavity in a femur.
 3. Themethod of claim 2 further comprising transecting a natural femoral headof the femur.
 4. The method of claim 3 wherein the step of transectingcomprises transecting the femoral head along a plane inclinedapproximately 70° to a longitudinal axis of the femur.
 5. The method ofclaim 1 wherein the step of opening the cavity comprises opening acavity in a humerus.
 6. The method of claim 1 further comprisingconstructing the implant component and the stem separately, forming areleasable lock on the implant component and the anchor, and attachingthe implant component to the lock.
 7. A method for installation of aprosthetic implant comprising the steps of:a. providing a functionalimplant component adapted to remain in permanent contact with a bone; b.forming a portion of the component in contact with the bone so that bonyingrowth can attach directly thereto; c. opening a cavity in the bone;d. placing the functional component against the bone; e. providing ananchor having a nonresorbable core surrounded by a resorbable sleevewhich has a periphery formed and dimensioned for firmly engaging acavity defining surface of the bone while maintaining sufficient spacingbetween the core and the cavity surface to prevent bone from contactingthe core after a resorption of the sleeve; f. giving the core a lengthso that it extends over a major portion of a length of the sleeve; g.inserting the anchor into the cavity so that the anchor is substantiallyimmovably disposed within the cavity; h. fixedly securing the componentto the anchor so that the component is in contact with bone surroundingthe cavity; i. growing bone proximate the cavity into the portion of thecomponent to thereby permanently fix the component to the bone; and j.substantially completely resorbing the sleeve after the completion ofthe promoting step so that natural tissue can grow back into the cavitywithout being able to reach the core; whereby the core remainspermanently in the cavity and remains spaced from the surface of thecavity after the resorption of the sleeve so that there is substantiallyno load transference between the bone and the core.
 8. A method forinstallation of a prosthetic implant comprising the steps of: providinga permanent, functional implant component; opening a cavity in the bone;providing an anchor a nonresorbable core surrounded by a resorbablesleeve, the sleeve having a given length extending into the cavity andthe core extending over a major portion of the given length of thesleeve, the sleeve having a wall thickness selected so that when theanchor is placed in the cavity a surface of the sleeve snugly engagesthe bone surrounding the cavity and thereby fixes the anchor relative tothe bone, the thickness of the sleeve being further selected to maintaina spacing between the bone and the core which is sufficient to preventthe bone, following a resorption of the sleeve, from growing intocontact with the core to prevent bony ingrowth into the core; insertingthe anchor into the cavity to substantially immovably position ittherein; securing the component to the anchor so that a portion of thecomponent is in contact with the bone in a vicinity of the cavity;growing bone into the portion of the component to thereby permanentlyfix the component to the bone; and resorbing the sleeve so thatthereafter the core remains nonfunctionally in the cavity while bonyingrowth into the component permanently secures the component to thebone.
 9. A method according to claim 7 wherein the portion of thecomponent is a surface oriented transversely to a longitudinal axis ofthe cavity, wherein the anchor extends in a direction of the cavity awayfrom the component surface, and wherein the sleeve covers at least amajor portion of the core from a free end of the core towards thecomponent surface.
 10. A method according to claim 8 wherein the step ofopening includes the step of resecting the bone in a vicinity of thecavity to form a bone surface complementary to the portion of thecomponent.
 11. A method according to claim 10 wherein the bone surfacewhich is complementary to the portion of the component has an anglerelative to the longitudinal axis of the cavity other than 90°.